1. Field of the Invention
The present invention relates to an agent and a process for the desulfurizing of pig iron and cast iron.
2. Information Disclosure Statement
The desulfurization of iron melts by using magnesium as a highly effective agent has become standard practice in the steel industry. Because of the violent reaction of the magnesium in the iron melt, it cannot be applied alone; it requires blending to reduce the strong agitation or mixtures of magnesium. For these blends, lime, fluorspar and slag residue from the aluminum smelting process has been used, as well as calcium carbide, etc. These finely granulated blends are injected through a lance into the melt.
Pneumatic feeding of these mixtures caused in general a problem of discontinuous and pulsating feeding of the agent, leading to an increase of agent used per unit. To overcome this it was proposed to coat the magnesium granule to achieve an improved fluidity of the mixture. But these coatings introduced usually unwanted additional slag components.
According to another proposal, magnesium is included in a hollow wire, which then is shot into the melt. Because of the high costs, this system is used only in special cases and small ladles.
Finally the segregation of the magnesium in the mixture causes another problem during the transportation or during handling. This leads to zones rich and poor in magnesium. Such blends lead to inconsistent and not forseeable final sulfurs in the treated melts. Further, it was proposed to use the same coating for magnesium and the dilutant. But such a procedure is expensive and therefore not accepted. These systems lead to dissolution of magnesium in the iron melt.
Another route is the so called co-injection, in which magnesium is injected with one dispenser and the diluting components with a second dispenser, both feed-streams mix in the lance; a certain ratio between both components is maintained. But this equipment is very complicated and expensive; it requires costly instrumentation. Also, these systems lead to a solution of magnesium with rather limited desulfurization. To overcome this, a sequential injection has been proposed, i.e. carbide and magnesium is injected in alternate cycles. At the end of the treatment, the dissolved magnesium is blown out from the melt with calcium carbide. Also, this system needs a highly instrumented plant.
Magnesium exhibits a post-desulfurizing property, after the injection has been finished further Mg-sulfides are precipitated; the stable final sulfur is achieved relatively late and incomplete.
All these processes lead to an unavoided loss of iron. Up to 3% of the melt weight is included in the dry or pasty slag. Fluorspar may improve this phenomenon, but because of environmental reasons, it is not accepted in general.
There was a demand for a desulfurizer for iron melts which avoids these drawbacks completely or at least to a great extent and one that is available at a favorable price. Above this was a permanent demand to use a dilutant which not only works as such but also contributes to the desulfurizing process.
It is therefore an object of this invention to provide an improved agent for the desulfurization of iron melts, which avoids the drawbacks of the state of the art.